Without limiting the scope of the invention, its background is described in connection with membranes for water purification. The membranes are commonly used in purification of water whether it is surface water (rivers and lakes), or underground water (aquifers) or industrial and municipal wastewater (mining, metals, dyeing, chemical) or produced water from oil and gas extraction as well as desalination (brackish or seawater). Membranes are very efficient and energy friendly for water purification, however, they suffer from fouling.
Fouling occurs when certain impurities in water deposit on a membrane's surface or in its internal pore structure. This deposition leads to a dramatic reduction in permeate flux, requiring periodic chemical cleanings resulting in increased operating costs and decreased membrane life. New membrane materials and treatments are researched to help reduce foulant adhesion. Recently, very thin coatings of polydopamine, polydopamine+PEG [Freeman et al. U.S. Pat. No. 8,017,050 issued Sep. 13, 2011] and hydroquinone, catechol, or mixtures of hydroquinone, catechol, and/or polydopamine [Freeman et al. Non-provisional patent application Ser. No. 12/939,764] onto the surface of commercial microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes have shown significant reduction in membrane fouling. A multi-year research program at the University of Texas resulted in filing of the above patents and patent applications beside a graduate thesis for Dr. Bryan McCloskey. Key finding of his research are published in a paper McCloskey et. al—“Influence of Polydopamine Deposition Conditions on Pure Water Flux and Foulant Adhesion Resistance of Reverse Osmosis, Ultrafiltration, and Microfiltration Membranes,” Polymer, 51, 3472-3485 (2010). In addition, more work on the subject matter was pursued by Xi, Z. Y and published as “A facile method of surface modification for hydrophobic polymer membranes based on the adhesive behavior of poly(DOPA) and poly(dopamine),” Journal of Membrane Science (2009). Details for the above works are incorporated herein as reference. It was demonstrated in previous works that beside antifouling properties, these thin polymeric coatings are extremely hydrophilic and permeable to water, however, these works either did not develop or disclosed details related to -1) improvements in membrane selectivity for ion rejection and their implications; -2) capability to effectively utilize the active chemistry use during the coating of polydopamine layer; -3) its storage, repeated and more effective use and safe disposal; and -4) effective maintainability and serviceability of the coated membranes. Advanced Hydro Inc. undertook the commercialization of the technology of the issued patent U.S. Pat. No. 8,017,050 and through additional research developed claims embodied in this patent application.
Since membranes are engineered and highly optimized to remove specific material from the feed effluent, for example, total dissolved solids (TDS) in form of mono and divalent ions such as NaCl, MgSO4, CaSO4, or other high molecular weight materials such as sugars, chemicals, organic molecules and other macromolecules. An RO membrane is optimized to reject NaCl for effective desalination while an NF membrane is optimized to reject divalent ions; however, the rejection of NF membranes for monovalent ions is significantly less than that of typical RO membranes. Similarly, UF membranes are designed to reject higher molecular weight impurities and become completely permeable to mono and divalent ions. While optimizing a rejection threshold for ion size or molecular weight cut-off, other properties such as permeability and energy efficiency are usually sacrificed.
In addition, membranes also wear over time due to exposure to undesired chemicals during cleaning or accidental exposure to free chlorine in the case of RO and NF membranes. It is highly desirable to have membranes with improved resistance to cleaning chemicals and free chlorine exposures used for maintenance of filtration systems. As cleaning chemicals impact the membrane performance over long term, it is desired to minimize the use of cleaning chemicals.
In this invention, we present details regarding how to exploit polymeric coating's high permeability to water with optimized coating conditions to make membrane more selective for ion-rejection while maintaining their energy efficiency. In addition, we present details regarding the ease of maintenance of the membranes with minimum use of chemicals as well as process that makes them more resistant to cleaning chemicals while harvesting the benefits of improved antifouling properties.